The process known as horizontal directional drilling is utilized to install a variety of underground utilities in a manner that does not disrupt the surface. In use, a drill machine is used drill a pilot bore that extends beneath the ground surface from an entry hole at the ground surface (i.e., a starting point) to an exit hole at the ground surface (i.e., an ending point). The pilot bore is drilled by rotating and pushing a ground engaging tool (e.g., a drill bit) that is attached to the end of a drill rod. The length of the pilot bore is extended by stringing multiple rods together to form a drill string. The direction of drilling can be controlled (i.e., the drill string can be “steered”) by various techniques to control the depth of the pilot bore as well as the location of the exit hole. The location of the drill string, after the pilot bore is completed, represents the desired location of the utility to be installed.
After the pilot bore is drilled, the drill bit is typically removed and a second ground engaging tool installed onto the end of the drill string. This tool is typically known as a backreamer. Its function is to ream the drilled bore to a diameter sufficient to allow installation of the utility. To provide a reaming function, the backreamer is typically pulled back through the pilot bore by the drill string as the drill string is withdrawn from the pilot bore. Often times the utility being installed is attached with a swivel located at the end of the backreamer such that the utility is pulled into the reamed bore immediately behind the backreamer. In this way, the act of withdrawing the drill string will simultaneously result in the installation of the utility.
The type of utilities installed typically includes telecommunications, power, water, natural gas, liquid gas pipelines, potable water pipes and sewers. Due to this large variety of utilities, there is a large variety in the size requirements for the final reamed borehole, and thus a wide range of backreamer sizes is required.
Different styles of backreamers are typically used for different soil conditions. A backreamer, for instance, designed to operate effectively in a sandy soil, will not operate effectively in a heavy clay. Backreamers capable of boring through rock are significantly different than those used for either sandy soils or clay. In the situations where the borehole passes through rock, multiple passes of backreamers of sequentially larger diameter may be required to achieve the desired final borehole size. Examples of various backreamers can be found in; U.S. Pat. No. 6,250,403; U.S. Pat. No. 5,921,331; U.S. Pat. No. 5,687,807; U.S. Pat. No. 4,754,526; U.S. Pat. No. 5,220,964; U.S. Pat. No. 5,390,750.
The cutting elements of back reamers often experience high wear rates, while other portions of the back reamers are not exposed to equivalent wear conditions. Thus, modular backreamers have been developed to minimize repair costs. Examples of such backreamers are disclosed in US20020108785; US20020088649; U.S. Pat. No. 6,386,298; U.S. Pat. No. 5,979,574; and U.S. Pat. No. 5,979,573.
Although various types of cutting elements are used, many back reamers for boring rock utilize rolling cutters mounted on bearings. Several designs have been developed to minimize the costs of maintaining these rolling cutters, examples are disclosed in U.S. Pat. No. 4,509,607; and U.S. Pat. No. 6,708,786.
In order to be able to adapt in a timely manner, the operator of the drill machine needs a variety of backreamers on-hand. The cost of each individual backreamer is significant, thus the inventory costs of backreamers is potentially substantial. There is a need for a backreaming system that provides improved flexibility with a reduced investment.